May I suggest adding a blurb about color contrast with the background image?
When the image has many colors or has gradients, it can be difficult to read text in front of it. Say for instance you have black text, and your image is a bright white sky above a dark foreboding cave. If the image is static, people reading the text over the dark section of the image may have difficulty making out the text, whereas it will show plainly on the light part of the image.
There are ways around it, like giving your text a single-color background of its own in front of the image. But for the most part, I'd just want designers and developers to be mindful of the potential problem. Could solve a lot of headaches.
Thanks for this guide, it's pretty boss.